Acetolysis of waste polyethylene terephthalate for upcycling and life-cycle assessment study

To reduce environmental pollution and reliance on fossil resources, polyethylene terephthalate as the most consumed synthetic polyester needs to be recycled effectively. However, the existing recycling methods cannot process colored or blended polyethylene terephthalate materials for upcycling. Here we report a new efficient method for acetolysis of waste polyethylene terephthalate into terephthalic acid and ethylene glycol diacetate in acetic acid. Since acetic acid can dissolve or decompose other components such as dyes, additives, blends, etc., Terephthalic acid can be crystallized out in a high-purity form. In addition, Ethylene glycol diacetate can be hydrolyzed to ethylene glycol or directly polymerized with terephthalic acid to form polyethylene terephthalate, completing the closed-loop recycling. Life cycle assessment shows that, compared with the existing commercialized chemical recycling methods, acetolysis offers a low-carbon pathway to achieve the full upcycling of waste polyethylene terephthalate.


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Post-processing: Terephthalic acid was separated by suction filtration, then washed with water several times and dried in a drying oven. We finally obtained 2350g TPA with a yield of 90.60% (purity: 99.86%).
The acetic acid in the reaction solution was spun out under reduced pressure, and the vacuum degree was controlled at 15±5mmHg.
Then, the remaining liquid (2578g) was distilled under reduced pressure by an oil pump, the vacuum degree was controlled in the range of 2-4 mmHg, the fractions at different temperatures were collected, and analyzed by GC. The GB/T 32685-2016 standard 5 ≤25 ≤150 The ASTM D7976-2020 International standard 6 ≤25 ≤190 These decolorized TPA can be seen in Fig. 3 in main text. n. d. = not detected.  by decolored method 1. We have tried the acetolysis of all PET fibers available around us, and all of them can achieve good results.

Supplementary
Supplementary Figure 11 Acetolysis of various PET fillers. Decolorized terephthalic acid obtained from different textiles yields

Supplementary Note 4: Process details and descriptions for Case 1 and Case 2 in ASPEN
In this simulation, poly-NRTL physical property method was selected to deal with PET depolymerization and polymerization process 7-10 (RBATCH1, CSTR1-2, RPLUG), At the same time, for other processes such as distillation, NRTL physical property method was used when dealing with acetic acid aqueous solution system, NRTL-HOC physical property method is used to accurately simulate.   is a re-polymerization stage. Since there is no model for polymerization between EGDA and TPA, we estimated this process based on the dynamic data from the polymerization process between EG and TPA. At the same time, new polymer segments and oligomers are redefined (Supplementary Table 24).

Supplementary Note 5: Process details and descriptions for the decolorization process in ASPEN
We simulate three decolorization methods for dark TPA. Figure 51): Activated carbon and crude TPA were added to the NaOH solution, and the container was heated to 110 ℃ for 20 mins. Here NaOH reacted with TPA to form terephthalic anions and Na + , which dissolved in water. Activated carbon with pigment was filtered and left behind. Since pigment components are difficult to define in Aspen, according to the final yield, 4% of TPA was treated as pigment which was trapped with the activated carbon. The filtrate was mixed with HCl solution to regenerate TPA, and purified TPA was obtained after filtration again.

Method 1 (Supplementary
The waste liquid was neutralized by NaOH and discharged. Figure 52): Activated carbon and crude TPA were added to the N, N-dimethylacetamide, and the container was heated to 150 ℃ for 20 mins. Similarly, 5% of TPA was trapped with activated carbon. TPA/DMAC adduct crystals were precipitated from the filtrate after freeze-recrystallization. (The energy required for cooling in this step was measured in electricity) After filtration and adding water, the adduct was dissociated to obtain TPA and DMAC.

Method 2 (Supplementary
DMAC was separated by distillation and reused. Figure 53): Activated carbon and crude TPA were added to the water, and the container was heated to 280 ℃ for 30 mins. 8% of TPA was trapped with activated carbon. After filtration and cooling, pure TPA is obtained.

Supplementary Note 6: Life-cycle assessment
The goal and scope of the LCA study in this work are shown in Supplementary Table 11. Besides, the system boundary of LCA was set as "cradle to gate" (i.e., collection and transportation of waste PET; Production of chemical raw materials; And the whole process of post-depolymerization and re-polymerization to produce recycled amorphous PET resin in forms of chips) because the subsequent use and disposal of various PET products vary widely. Since there is no data on the pretreatment process of waste PET plastic collected in the early stage, we can only assume that it is consistent with mechanical recycling 12 . At the same time, our distribution principle followed the "cut-off" rule that the first life cycle (original plastic) and the second life cycle (recycled plastic) are independent of each other. For inventory analysis, our material balance and energy balance were derived from actual experimental results and Aspen simulation results. These were our "prospective processes", and the functional unit was set at 1 kg of amorphous PET resin in forms of chips (PRO-PET in Aspen Plus). The impact assessment was carried out on professional software OpenLCA 1.10.3, and the assessment methods were IPCC AR5 for GWP, CML-IA baseline for NREU. "Background process" data were obtained from the database Ecoinvent V. 3 and Global Warming Potential (GWP, the result is expressed by kg CO2 equivalent/kg amorphous PET resin in forms of chips, namely kg CO2-eq per kg amorphous PET resin in forms of chips).
Note: In the mechanical shredding process, due to the high-water tolerance of the follow depolymerization system (i.e., complete depolymerization can be guaranteed even when the mass ratio of water to PET is up to 25%), the drying here means drain off instead of heating. In the extrusion process, the high-temperature PET resin from the reactor is directly involved in the granulation process, so there is no additional heat supply. In the decolorization process, the intercepted activated carbon can be recycled, which corresponds to the "Treatment of spent activated carbon" process in OpenLCA.
The result of this activity is 0.9 kg of reactivated carbon from 1 kg of spent activated carbon.  (Ⅱ) Non-renewable energy use (NREU), MJ;

Limitations
In addition to the above-mentioned assumptions, the following aspects are not assessed in this study: Plant construction and equipment maintenance.

Report requirements
To present the outcome via journal publication which is openly accessible to everyone.

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Supplementary Supplementary Table 13 Superscript index table of Supplementary Table 12 Interpretation Page a The NREU and GWP of mechanical shredding in Case 1 (EU) and Case 2 (EU) can be obtained in Supplementary Table 18  S41   b The NREU and GWP of mechanical shredding in Case 1 (CN) and Case 2 (CN) can be obtained in Supplementary Table 18  S41   c The NREU and GWP of acetolysis (Section 1) in Case 1 (EU) can be obtained in Supplementary Table 23-I  S48   d The NREU and GWP of acetolysis (Section 1) in Case 2 (EU) can be obtained in Supplementary Table 28-I  S53   e The NREU and GWP of acetolysis (Section 1) in Case 1 (CN) can be obtained in Supplementary Table 23-I  S48   f The NREU and GWP of acetolysis (Section 1) in Case 2 (CN) can be obtained in Supplementary Table 28-I  S53   g The NREU and GWP of EGDA hydrolysis (Section 2) in Case 1 (EU) can be obtained in Supplementary Table 23-II  S48   h The NREU and GWP of EGDA hydrolysis (Section 2) in Case 1 (CN) can be obtained in Supplementary Supplementary Table 15 Superscript index table of Supplementary Table 14 Interpretation Page a The NREU and GWP of mechanical shredding in Case 1 (EU) and Case 2 (EU) can be obtained in Supplementary Table 19  S41  b The NREU and GWP of mechanical shredding in Case 1 (CN) and Case 2 (CN) can be obtained in Supplementary Table 19  S41  c The NREU and GWP of acetolysis (Section 1) in Case 1 (EU) can be obtained in Supplementary Table 32-I  S61  d The NREU and GWP of acetolysis (Section 1) in Case 2 (EU) can be obtained in Supplementary Table 36-I  S66  e The NREU and GWP of acetolysis (Section 1) in Case 1 (CN) can be obtained in Supplementary Table 32-I  S61  f The NREU and GWP of acetolysis (Section 1) in Case 2 (CN) can be obtained in Supplementary Table 36-I  S66  g The NREU and GWP of decolorization in Case 1 (EU) can be obtained in Supplementary Table 32-II  S61  h The NREU and GWP of decolorization in Case 2 (EU) can be obtained in Supplementary Table 36-II  S66  i The NREU and GWP of decolorization in Case 1 (CN) can be obtained in Supplementary Table 32-II  S61  j The NREU and GWP of decolorization in Case 2 (CN) can be obtained in Supplementary Table 36-II  S66  k The NREU and GWP of EGDA hydrolysis (Section 2) in Case 1 (EU) can be obtained in Supplementary Table 32-III  S62  l The NREU and GWP of EGDA hydrolysis (Section 2) in Case 1 (CN) can be obtained in Supplementary Table 32-III  S61  m The NREU and GWP of repolymerization (Section 3) in Case 1 (EU) can be obtained in Supplementary Table 32-IV  S61  n The NREU and GWP of repolymerization (Section 3) in Case 2 (EU) can be obtained in Supplementary Table 36-III  S66  o The NREU and GWP of repolymerization (Section 3) in Case 1 (CN) can be obtained in Supplementary Table 32-IV  S61  p The NREU and GWP of repolymerization (Section 3) in Case 2 (CN) can be obtained in Supplementary Table 36-III  S66  q The NREU and GWP of extrusion in Case 1 (EU) / Case 2 (EU) can be obtained in Supplementary